A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to develop a practical lithographic resist process, it is desirable to first use a lithography simulation tool for process optimization. Subsequently, actual substrate printing verification may be performed. The challenge is how to ensure the prediction of a resist critical dimension (CD), a contour of the resist pattern and/or whether some contours of resist patterns are merged or cut with sufficient accuracy when using the simulation tool. Resist model calibration is one of the critical key factors in the simulation process, and the robustness of the simulation model is another important factor.
A typical lithography simulation involves three basic steps. First, an aerial image for the feature in question is calculated. The aerial image calculation is based on the optical settings of a lithographic apparatus, which include, for example, numerical aperture, exposure wavelength and characteristics of lenses. Second, a post exposure bake (PEB) step is performed. This step provides two functions: (1) allowing chemical amplification for photo speed to take place due to the heat, and (2) minimizing resist CD swing caused by standing wave effects. The third step involves developing a resist pattern based on the diffused aerial image.
In general, there are two conventional approaches to resist modeling: either through a slow but more physical modeling of the process or through a faster but empirical approach. However, none of these approaches to resist modeling provides satisfactory results. Indeed, these approaches may not fully represent the chemical processes and/or are generally time-consuming